1. Field of the Invention
The invention relates to a process for making a product with an easily cleanable surface by coating the surface with a hydrophobic material and to products obtained by this process.
2. Description of the Related Art
It is generally known to provide objects with soil-repelling substances. For example, it is known to treat the surface of glass, glass-ceramic or glazing, or even of a rock, with a silicone to make them soil-repellent and water-repellent. This is usually accomplished by rendering the surface of the objects to be treated hydrophobic by applying a liquid composition. Many chemicals, particularly silicone oils and/or fluorinated silanes, are commonly used for this purpose. The surfaces treated in this manner are difficult to wet, because the water beads up and runs off. Dirt adheres to the treated surfaces only slightly and can therefore be readily removed. This is particularly advantageous for outdoor use, because, for example in the case of skylights and/or glass roofs, such as those on winter gardens, etc, the deposited dirt is entrained and removed as a result of the rainwater beading up and running off. It is thus possible to keep such surfaces permanently clean without additional cleaning.
The drawback of this method is that the applied chemicals can form permanent bonds only by reacting with OH groups directly available on the substrate material. Sufficiently reactive OH groups are not present on the surface of the treated objects, particularly on glass, without appropriate pretreatment, for example with hydrogen/oxygen plasma. Thus this method gives rise only to a very thin, mostly monomolecular hydrophobic coating which during use, particularly owing to mechanical stress, such as cleaning and/or abrasion by wind and dust, is rubbed off quickly so that the desired self-cleaning property is lost.
Attempts have already been made to improve the durability of such coatings. For example, EP-A-0 658 525 describes the preparation of a water-repellent multilayer film involving the preparation of three different sol solutions which are then mixed and applied to a glass substrate forming a gel coating on the glass surface. Heating then gives rise to a superficial metal oxide layer. To this metal oxide layer is then applied a fluoroalkylsilane coating, as previously described.
JP-A-11 092 175 describes a process in which a methoxysilane or an ethoxysilane compound containing a fluorocarbon chain is attached to the surfaces of small particles with a diameter of 100 nm. The particles modified in this manner are then dissolved in an aqueous medium and applied to the surfaces to be coated. The solvent is then removed and the residue baked. This provides surfaces coated with small hydrophobic particles.
WO 99/64363 describes a method of preparing a water-repellent surface, which comprises first roughening the surface of the glass and removing all metal ions present on the surface. A water-repellent film is then applied in a known manner to the previously treated surface. The roughening of the surface makes it possible for the water repellant to fill the roughness valleys.
WO 99/02463 describes the preparation of a scratch-resistant coating, which involves applying an organic substance with a silicone-like network to a surface. This is followed by a heat treatment in which the temperature and duration are chosen so that the applied purely organic layer is substantially degraded and/or removed. However in the topmost molecular layer the inorganic molecules of the substrate and the organic molecules of the applied substance can form a compound. In this manner, an organic substance, for example a methyl group, is directly attached to the silicon atom of a glass surface by formation of a Si—C bond.
DE 695 02671 T2 (WO95/24053) describes a display device comprising a display screen provided with a light-absorbing coating comprising a hybrid inorganic-organic material consisting of an inorganic network of silicon oxide and metal oxide. The polymeric chains are intertwined with the inorganic network, thereby forming a hybrid inorganic-organic network. It has been shown, however, that organic components, particularly hydrophobic organic components, such as fluoroalkyls, cannot be incorporated homogeneously in such a layer, but that said components primarily adhere to the surface facing away from the carrier layer.
All hydrophobic and possibly soil-repellent properties conferred by the foregoing processes are not sufficiently durable in use and are rapidly lost, particularly as a result of mechanical stress.